Although mutator factors, including such elements as the male recombination (MR) factor in Drosophila melanogaster, occur in natural populations, we still have little information about their frequencies or the effects they have upon the genetic structure of populations. Our study has two primary sets of objectives. First, we are surveying Drosophila populations to determine the frequency of mutator factors, concentrating at first upon strains showing MR activity. MR activity was found in all of the ten populations we have sampled, and was found in almost all individuals in each population. It is, therefore, common. Second, we attempted to understand the mechanism by which the MR mutator factor produces its effects. Whole chromosome substitutions showed that the factor is associated with the second, and to a lesser extent, the third chromosome, and a difference between reciprocal crosses exists. No interchromosomal effect was found. Thus, the mechanism of male and female recombination were found to differ. Male recombination in the Mr strains apparently involve some premeiotic trigger event and can occur in clusters. Spermatocytes from males heterozygous for a paracentric inversion contained chromosome bridges, fragments, and signs of chromosome pulverization. Preliminary experiments suggest that the MR factor is infectious when fed or injected into non-MR strains. This supports our current hypothesis that this common mutator factor is a micro-organism that can become associated with, and break, chromosomes in natural outcrossed populations, causing apparent male recombination by chromosome breakage and reunion.